The present invention relates to a substrate suitable for use in the production of photopolymerisable elements and to the photopolymerisable elements produced from the substrates.
Relief printing plates formed from photopolymerisable elements comprising a transparent thermoplastics film support are described in British patent specifications Nos. 1,321,108 and 1,507,660. Such printing plates are produced by exposing the photopolymerisable element comprising a photopolymerisable layer carried by the film support to actinic light through a master such as a photographic image. After the exposure step, the non-light-struck areas of the photopolymerisable layer are washed away by treatment with a suitable solvent to leave a photopolymerised relief printing pattern in the light-struck areas.
The adhesion of the relief printing pattern to the film support should ideally resist damage during repeated use, e.g. in rotary printing machines.
The present invention relates to a substrate suitable for use in the production of a photopolymerisable element comprising an adhesive layer which is capable of promoting tenacious adhesion between the film support and the photopolymerisable relief areas formed by exposing to light a photopolymerisable layer applied over the adhesive layer.
According to the present invention, a substrate suitable for the production of a photopolymerisable element, comprises a self-supporting plastics film or sheet having an adhesive layer superimposed upon at least one surface thereof, wherein the adhesive layer comprises an unsaturated resin having the formula: ##STR2## wherein R.sub.1 and R.sub.2 are the same or different and represent lower alkyl groups having up to 6 carbon atoms, x and y are the same or different and are an integer from 1 to 4 inclusive and n is an integer from 4 to 30 inclusive.
According to a further aspect of the invention, a photopolymerisable element comprises the substrate defined above and a photopolymerisable layer superimposed in interfacial contact upon the adhesive layer of the substrate.
The invention also relates to relief printing plates derived from the photopolymerisable element according to the invention and comprising a relief printing pattern formed in the photopolymerisable layer.
The lower alkyl groups R.sub.1 and R.sub.2 of the unsaturated resin structure are preferably selected from methyl or ethyl and also are preferably the same. It is especially preferred that both R.sub.1 and R.sub.2 are methyl groups.
The integers x and y of the resin structure are preferably 2 or 3 and are preferably both either 2 or 3. Most preferably x and y are both equal to 3.
The integer n is preferably from 8 to 12 inclusive.
It will be appreciated that the unsaturated resins may be derived from maleic or fumaric acid although it is preferred that the resin should be a fumarate condensate.
The preferred unsaturated resin is a propoxylated bisphenol A-fumarate condensate.
The unsaturated resin preferably has a molecular weight in the range 2000-10,000 and most preferably 3000-4000.
If desired, a photopolymerisation initiator or a mixture of initiators may be included in the adhesive layer, preferably in an amount 0.01 to 10% by weight based upon the weight of the unsaturated resin, to enhance the adhesion between the adhesive layer of the substrate and the photopolymerisable layer applied thereto in the production of the photopolymerisable element, e.g. as described in British patent specification No. 1,321,108. Photopolymerisation initiators suitable for inclusion in the adhesive layer may be chosen from benzoin, benzoin isopropyl ether, benzoin isobutyl ether, benzoin methyl ether, benzoin ethyl ether, .alpha.-methyl benzoin, benzil, anthraquinone, diacetyl anthraquinone, 1-chloroanthraquinone, 9,10-phenanthraquinone, diphenyl disulphide, 2-naphthalenesulphonyl chloride, .omega.-bromoacetophenone, eosin, thionin, and benzophenone.
Photoinitiator mixtures suitable for inclusion in the adhesive layer may comprise an admixture of:
(i) a compound having the formula: ##STR3## wherein R.sub.3 and R.sub.4 are the same or different and are selected from hydrogen, or a lower alkyl group having up to 6 carbon atoms and R.sub.5 is the same as or different from R.sub.3 or R.sub.4 and is selected from hydrogen, a lower alkyl group having up to 6 carbon atoms, a partially or completely halogenated alkyl group having up to 6 carbon atoms or a group of the formula: ##STR4## in which R.sub.6 and R.sub.7 are the same or different and are selected from the same substituents as R.sub.3 and R.sub.4, said compound being present in an amount 0.01 to 2.0% by weight based upon the weight of the unsaturated resin; and
(ii) another photopolymerisation initiator preferably chosen from those listed above, present in an amount 0.1 to 7% by weight based upon the weight of the unsaturated resin.
R.sub.3 and R.sub.4 of compound (i) are preferably lower alkyl groups having up to 6 carbon atoms and most preferably both are methyl groups. R.sub.5 is preferably hydrogen, CF.sub.3 or: ##STR5## in which R.sub.6 and R.sub.7 are lower alkyl groups having up to 6 carbon atoms and most preferably both are methyl groups. Compound (i) may be: ##STR6## or most preferably 4,4'-bis(dimethylamino)-benzophenone, i.e. ##STR7##
The amount of the photopolymerisation initiator (i), especially 4,4'-bis(dimethylamino)-benzophenone, in such an admixture is in the range 0.01 to 1.0% and preferably 0.01 to 0.5% by weight based upon the weight of the resin.
The photopolymerisation initiator (ii) is preferably present in the admixture in an amount in the range 0.25 to 5.0% by weight based upon the weight of the resin.
A preferred photopolymerisation initiator mixture for use in the adhesive layer of the invention includes isopropyl benzoin ether and 4,4'-bis(dimethylamino)-benzophenone. Such an admixture is particularly effective when it includes about 5.0% by weight of isopropyl benzoin ether and about 0.25% by weight of the 4,4'-bis(dimethylamino)-benzophenone, both amounts being based upon the weight of the resin.
The adhesive layer may include up to 10% by weight based upon the weight of the resin of one or more inert particulate materials to improve such properties as slip and resistance to blocking which are important for the general handling and reeling properties of the film substrates to which the adhesive layer is applied. Suitable particulate materials include inorganic particles such as particles of silica and alumina hydrate having an average particle size in the range 0.1 to 30 .mu.m.
The adhesive layer is preferably applied to the surface of the film or sheet as a solution in an organic solvent such as a ketone, a chlorinated hydrocarbon, or an alcohol, e.g. acetone, methyl ethyl ketone, methylene chloride or methanol, by any suitable known coating technique. After application from such a solution, the adhesive layer is preferably dried to a dry coating thickness of from 0.1 to 15 .mu.m, and most preferably from 0.2 to 8 .mu.m. Thicknesses less than 0.1 .mu.m provide poor adhesion to the film whilst thicknesses exceeding 15 .mu.m may incur cohesive failure within the adhesive layer.
The self-supporting plastics film or sheet may consist of any suitable plastics material such as films or sheets of cellulose esters, e.g. cellulose acetate, polystyrene, polyamides, polymers and copolymers of vinyl chloride, polycarbonate polymers and copolymers of olefines, e.g. polypropylene, polysulphones and linear polyesters which may be obtained by condensing one or more dicarboxylic acids or their lower alkyl diesters, e.g. terephthalic acid, isophthalic, phthalic, 2,5-, 2,6- and 2,7-naphthalene dicarboxylic acid, succinic acid, sebacic acid, adipic acid, azelaic acid, diphenyl dicarboxylic acid, and hexahydroterephthalic acid or bis-p-carboxyl phenoxy ethane, optionally with a mono-carboxylic acid, such as pivalic acid, with one or more glycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and 1,4-cyclohexane-dimethanol. Biaxially oriented and heat-set films of polyethylene terephthalate are particularly useful for the production of the film supports according to this invention and may be produced by any of the processes known in the art, e.g. as described in British patent specification No. 838,708.
The surface of polyester film supports may be subjected to an adhesion-promoting pretreatment prior to coating with the adhesive layer. The pretreatment may preferably involve subjecting the film surface to the action of a material having a swelling or solvent action upon the film, e.g. a solution in a common organic solvent such as acetone or methanol of 0.5 to 20% by weight of p-chloro-m-cresol, 2,4-dichlorophenol, 2,4,6- or 2,4,5- trichlorophenol or 4-chlororesorcinol or a mixture of such materials in a common organic solvent. If desired, the pretreating solution may also include an adhesion-promoting resin such as a partially hydrolysed or unhydrolysed copolymer of vinyl chloride-vinyl acetate. After such a solution has been applied to the film surface it is dried at an elevated temperature for a few minutes before applying the adhesive composition.
Alternatively, the pretreatment step may be omitted and the material having the swelling or solvent action on the polyester film may be included in the adhesive composition, the amount of the swelling or solvent component comprising approximately 5 parts by weight to one part by weight of the copolymer component.
The photopolymerisable element may be produced by the printing plate manufacturer by casting a liquid photopolymerisable layer upon the adhesive layer which has already been applied to the film or sheet. Alternatively, the production of the printing plates may commence with a photopolymerisable element as defined above, i.e. an element comprising the film or sheet, the adhesive layer and the photopolymerisable layer.
The photopolymerisable layer may comprise an organic unsaturated material which is capable of being polymerised in the presence of a photopolymerisation initiator upon exposure to active radiation generally in the wavelength range 2000 to 8000 Angstroms and a suitable photopolymerisation initiator. The photopolymerisable organic unsaturated material may include an ethylenically unsaturated addition polymerisable monomer and/or an unsaturated polymer. Suitable ethylenically unsaturated addition polymerisable monomers, as described in British patent specification No. 1,321,108, include styrene, chlorostyrene, vinyltoluene, diallylphthalate, diallylisophthalate, triallylcyanurate, N,N'-methylenebisacrylamide, N,N'-hexamethylenebisacrylamide, stearyl acrylate, acrylamide, methacrylamide, N-hydroxyethylacrylamide, N-hydroxymethacrylamide, .alpha.-acetamido-acrylamide, acrylic acid, methacrylic acid, .alpha.-chloroacrylic acid, p-carboxystyrene, 2,5-dihydroxystyrene, methyl acrylate, methyl methacrylate, n-butyl acrylate, isobutyl acrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, trimethylene glycol diacrylate, trimethylene glycol dimethacrylate, tetraethylene glycol diacrylate and tetraethylene glycol dimethacrylate.
Suitable unsaturated polymers include unsaturated polyesters and unsaturated polyurethanes. Such unsaturated polyesters may be obtained by reacting diols such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyoxyethylene glycol of the formula: EQU HO--(CH.sub.2 --CH.sub.2 --O).sub.n --H
wherein n is an integer of 5 to 50, propylene glycol, dipropylene glycol, polyoxypropylene glycols of the formula: ##STR8## wherein m is an integer of 3 to 30, polyoxybutylene glycols of the formula: EQU HO(CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 --O).sub.x --H
wherein x is an integer of 2 to 30 and copolyglycols comprising --(CH.sub.2 --CH.sub.2 --O).sub.a -- groups and --(CH.sub.2 --CH--O).sub.b -- groups, wherein a and b, which may be the same or different, are integers from 2 to 50, with unsaturated dicarboxylic acids such as maleic acid, fumaric acid, itaconic acid, mesaconic acid, glutaconic acid, or the esters or the anhydrides thereof, for example, the dimethyl or diethyl esters or maleic anhydride.
Such unsaturated polyurethanes may be obtained by reacting unsaturated polyesters having terminal hydroxyl groups with polyisocyanates such as tolylene di-isocyanate and diphenylmethane di-isocyanate. The unsaturated polyesters having terminal hydroxyl groups are produced by the polycondensation reaction of the above-mentioned unsaturated dicarboxylic acids and excess diols mentioned above.
A photopolymerisation initiator is preferably included in the photopolymerisable layer and may consist of one or more of those initiators listed above as being suitable for inclusion in the adhesive layer. The amount of such photopolymerisation initiator used in the photopolymerisable material is preferably in the range of from 0.001 to 10% by weight based upon the total weight of the photopolymerisable composition. If desired, a photopolymerisation initiator having the structure of the initiator (i) of the adhesive layer may optionally be included in the photopolymerisable layer.
Known stabilisers may be employed for the purpose of maintaining storage stability (shelf life) of the photopolymerisable material. Exemplary stabilisers include hydroquinone, mono-tert-butyl hydroquinone, benzoquinone, 2,5-diphenyl-p-benzoquinone, pyridine, phenothiazine, p-diaminobenzene, -naphthol, naphthylamine, pyrogallol, cuprous chloride and nitrobenzene. These stabilisers are added only for completely preventing polymerisation reaction in the absence of actinic radiation such as that used in the production of printing plates. The amount of the stabilisers may preferably be about 0.005 to 3.0% by weight of the total weight of the photopolymerisable composition.
Fillers or binders may also be included in the photopolymerisable layer. Examples of suitable binders include cellulose esters such as acetyl cellulose, cellulose acetate, hydrogen succinate, nitrocellulose, cellulose acetate propionate and cellulose butyrate, polyvinylalcohol derivatives such as polyvinylbutyral, polyamide such as poly-.epsilon.-caprolactam, polyhexamethylene sebacamide, copoly(.epsilon.-caprolactamhexamethylene sebacamide and/or adipamide), and of suitable fillers include glass, mica, potter's clay, silica, asbestos, magnesium silicate, magnesium carbonate, alumina and polyethylene terephthalate in powder form.
The photopolymerisable layer may be photopolymerised by actinic radiation having wavelengths generally between 2000 and 8000 Angstroms. Practical sources of such actinic radiation include carbon arc lamps, super high pressure mercury lamps, high pressure mercury lamps, low pressure mercury lamps, UV fluorescent lamps, xenon lamps and sunlight.
In one preferred embodiment of the invention, relief printing plates are produced from the photopolymerisable elements by a process which involves exposing the element to actinic light from each side of the element in sequence. The self-supporting plastics film or sheet for use in such an element should be transparent to the wavelength of light employed for the photopolymerisation.
The photopolymerised layers are retained with excellent adhesion to the film or sheet via the adhesive layer and exhibit a remarkably sharp definition and excellent removal of the uncured regions of the photopolymerisable layer.
The configuration of a substrate and photopolymerisable element according to the invention is described below with reference to the accompanying drawings together with the production of a photorelief printing plate from the element.